Improvements in manufacturing technology and materials are the keys to increased performance and reduced costs for many articles. As an example, continuing and often interrelated improvements in processes and materials have resulted in major increases in the performance of aircraft gas turbine engines. Many of the improvements in gas turbine engines has been in extending the life expectancy of the turbine components that are subjected to cyclic stresses at high temperatures in corrosive, oxidative environments. Extending the life expectancy increases the mean service life of the components before replacement or repair. This is a benefit to the operator of the aircraft that uses the gas turbine engines, as less “down time” for repairs or replacements is experienced, resulting in an increase in operational time for revenues service.
Many of the parts used in the hot section of the turbine engine, such as turbine airfoils, are coated with thermal barrier coating systems. These systems are subject deterioration as a result of the extreme service conditions in a hostile environment. Constant improvements are sought in the life expectancy of thermal barrier coating systems to improve the performance of the engine. These thermal barrier coating systems include high temperature superalloys that are coated with high temperature ceramic materials to improve their temperature capability. In order to improve the adhesion of the ceramic material applied as a coating to the substrate, a bond coat is placed between the substrate and the ceramic top coating. Various bond coats have been used including MCrAlY's and diffusion aluminides.
Preoxidation of MCrAlY bond coats prior to application of a thermal barrier layer to form a thin alumina layer by in-situ oxidation of the MCrAlY layer using a commercially pure hydrogen atmosphere at 1975° F. has been found to improve the adhesion of the ceramic layer as set forth in U.S. Pat. No. 4,414,249 to Ulion et al. The preoxidation is preceded by polishing of the MCrAlY. While the polishing step was important, the technique was not felt to be important and could be accomplished mechanically electrochemically or chemically.
U.S. Pat. No. 5,238,752 to Duderstat et al. teaches the formation of a thin coating of alumina over a diffusion aluminide coating during the heating step which is part of the deposition process for the ceramic coating. Heating to a temperature of about 1800° F. is accomplished in a vacuum chamber in an atmosphere of about 5×10−5 Torr to produce a coating thickness of about 2×10−5 inches.
Others have discussed similar formation of a protective alumina coating by using a low partial pressure oxygen atmosphere during thermal treatment. Exemplary teachings include U.S. Pat. No. 5,716,720 to Murphy, which teaches formation of the alumina layer by evacuating a vacuum furnace to 10−6 Torrs before backfilling the furnace with Ar during a heat treatment of 1975° F. Rickerby et al. in U.S. Pat. No. 5,763,107 teaches the formation of the alumina layer during the formation of the ceramic coating after evacuation at a pressure of about 10−5 Torr, the dissociation of the ceramic during the EB application of the zirconia contributing to the oxidation of the bond coat. Rickerby et al. also teaches the optional intentional addition of oxygen during application of the ceramic top coat.
Myriad other teachings of formation of an alumina over a bond coat, whether a MCrAlY or a diffusion aluminide or a superalloy substrate exist. Even when protective atmospheres are formed, the formation of an alumina layer thus appears to be incidental to subsequent high temperature exposure of an outer layer that contains a perceivable quantity of aluminum, which is a fundamental part of the composition of MCrAlY diffusion aluminides and superalloy substrates. These general approaches describe the formation of a “pure” alumina by CVD deposition or its thermal growth under carefully controlled atmospheres to achieve oxide thicknesses of 0.25-25 microns.
These teachings indicate that there are a number of considerations in the formation of oxide over the layer underlying the ceramic top coat. Among these considerations is the completion of oxide phase transformation from transient oxides to a dense alpha alumina. Also to be considered is the purity of the oxide formed. The kinetics of the transition from internal to external oxidation as well as the formation of a “defect free” contiguous layer may also contribute to the success of oxide formation in a particular system. Other system-specific factors may include the columnar oxide grain structures and oxide growth by inward diffusion with no lateral oxide growth.
Despite the large number of teachings of the benefit of the formation of an alumina layer over a bond coat prior to the application of a ceramic top coat, and the perceived novelty of forming such alumina layers using various processing techniques, the formation of an alumina layer is incidental to subsequent high temperature exposure of an outer layer that includes some aluminum. It is clear, however, that there are other factors that contribute to the inevitable and incidental formation of alumina at elevated temperatures, and that the complexity of the formation of this alumina in varying and different compositions is not, and has not been, well understood. To achieve a “good” alumina coating, the complexity of the process of oxide film formation must be taken into consideration, along with other seldom discussed and interrelated factors such as the chemical composition of the surface that will underlie the ceramic top coat, whether substrate material or a subsequently applied bond coat, coating microstructure as well as surface conditions. The formation of acceptable alumina is very specific to the coating system and the interrelationship of these factors.
What is needed are advanced coatings in which these various interrelated factors are understood so that coating systems can yield longer life expectancies or higher temperature capabilities.